Many neurological disorders are associated with genetic mutations that affect neuronal activity and synapse function. Understanding how these genes regulate normal circuit function will have profound impact on the management of such diseases. In addition to neurons, the brain contains nearly ten times as many non-neuronal glial cells, which support neuronal function and regulate excitation/inhibition balance. The studies of mammalian model systems are hindered by this cellular and genetic complexity of the mammalian brain. The use of a simple, whole organism model system has the advantages of reducing the cellular complexity, while maintaining the neuronal and non-neuronal connectivity under physiological conditions. The overall goal of this project is to uncover the mechanisms by which non-neuronal cells modulate neuronal excitation/inhibition balance. The roundworm, Caenorhabditis elegans, will be utilized as a model system for four main reasons: 1) its neuronal networks are formed and maintained through mechanisms that are conserved in humans, 2) it has a simple, fully mapped nervous system, 3) it is easy to manipulate through genetic techniques, and 4) it has well-conserved homologs to genes mutated in autism spectrum disorders and epilepsy. The goals of this study will be accomplished through the following specific aims: Aim 1: Identify genetic pathways in non-neuronal cells that regulate neuronal excitation/inhibition imbalance using an RNA- interference screen. Aim 2: Characterize the physical interactions between neurons and non-neuronal cells under excitation/inhibition imbalanced conditions utilizing a genetic approach to fluorescently tag cellular interactions. Aim 3: Determine whether modulation of non-neuronal cells can prevent excitation/inhibition imbalance caused by mutations in autism spectrum disorder genes. The completion of this application will provide a deeper understanding of the interactions between neurons and the surrounding non-neuronal cells under physiological and pathological conditions. Additionally, this study will uncover the pathogenic mechanism(s) of neurological disorders that affect synaptic functions, such as autism spectrum disorders or epilepsy. Finally, this project will provide potential targets for novel therapies for the treatment of autism spectrum disorders, epilepsy, and related neurological diseases.